Method for electrolyzing molten salt, electrolytic cell, and process for producing ti using said method

ABSTRACT

The present invention provides a method for electrolyzing molten salt that can enhance the concentration of metal-fog forming metal in the molten salt by carrying out the electrolysis under conditions that the molten salt containing the chloride of metal-fog forming metal is supplied from one end of an electrolytic cell to a space between an anode and a cathode in a continuous or intermittent manner to provide a flow rate in one direction to the molten salt in the vicinity of the surface of the cathode and thus to allow the molten salt to flow in one direction in the vicinity of the surface of the cathode. According to the present invention, while high current efficiency is maintained, only the molten salt enriched with metal-fog forming metal such as Ca can be effectively taken out. Further, this method can easily be carried out by using the electrolytic cell according to the present invention. Furthermore, the application of the method for electrolyzing molten salt according to the present invention to the production of Ti by Ca reduction can realize the production of metallic Ti with high efficiency. Thus, the method for electrolyzing molten salt, the electrolytic cell, and the process for producing Ti, each according to the present invention, can be effectively utilized in the production of Ti by Ca reduction.

TECHNICAL FIELD

The present invention relates to a method for electrolyzing molten salt by which molten salt increased in Ca concentration by electrolyzing molten salt containing the chloride of metal-fog forming metal (e.g. Ca, Li, Na, Al, etc.), in particular CaCl₂, can be obtained, an electrolytic cell for use in carrying out that method, and a process for producing Ti using that method.

BACKGROUND ART

A common industrial process for producing metallic Ti is the Kroll process comprising reducing TiCl₄ with Mg. In this Kroll process, metallic Ti is produced via a reduction step and a vacuum separation step. In the reduction step, liquid-form TiCl₄ fed from above in a reaction vessel is reduced by molten Mg, whereupon granular metallic Ti is formed and then gradually moves downward and settles to give metallic Ti in sponge form. In the vacuum separation step, the unreacted Mg and the byproduct MgCl₂ are removed from the metallic Ti in sponge form inside the reaction vessel.

In the production of metallic Ti by the Kroll process, it is possible to produce high-purity products. However, since the process is a batch-wise one, the production costs increase and the prices of the products become very high. One of the causes for the increased production costs is the difficulty in increasing the rate of feeding of TiCl₄.

While several reasons therefor are conceivable, one is that when the rate of feeding of TiCl₄ is excessively increased, TiCl₄ is supplied from the above onto the MgCl₂ failing to move downwards and remaining on the liquid surface and, therefore, the TiCl₄ fed is partly discharged from the reaction vessel in the form of unreacted TiCl₄ gas and/or insufficiently reduced TiCl₃ gas, among others; resulting in a reduced efficiency in utilization of TiCl₄.

Further, in the Kroll process, the reaction is carried out only in the vicinity of the liquid surface of the molten Mg in the reaction vessel, so that the heat-liberating area is narrow. Therefore, cooling will not be able to keep up with the supply of TiCl₄ if fed at a high rate; this is also a major reason for the rate of feeding of TiCl₄ being limited.

Further, due to the wettability (stickiness) of molten Mg, the formed Ti powder moves downwards in a flocculated state and even during moving downwards, sintering and resulting grain growth occur by the heat which the high-temperature molten liquid has, thus rendering it difficult to discharge the same out of the reaction vessel. As a result, the production of metallic Ti cannot be carried out continuously, and the productivity is fettered.

As regards another process for producing Ti other than the Kroll process, the specification of U.S. Pat. No. 2,205,854 describes that Ca can be used as a reducing agent other than Mg for reduction of TiCl₄. And, as a process for producing Ti using the reduction reaction with Ca, a method is described in the specification of U.S. Pat, No. 4,820,339 (hereinafter referred to as “Document 1”) which comprises retaining CaCl₂ in molten salt form in a reaction vessel, feeding a metallic Ca powder into the molten salt from the above and allowing the Ca powder to dissolve in the molten salt and, at the same time, supplying TiCl₄ gas from the below for causing the molten Ca to react with TiCl₄ in molten CaCl₂ salt.

However, the process described in Document 1 cited above cannot become effective as a commercial process for producing Ti since the metallic Ca powder to be used as the reducing agent is very. expensive and, if this is purchased and used, the production costs will become higher as compared with the Kroll process. In addition, it is very difficult to handle Ca which is highly reactive; this is also an important factor hindering the industrial use of the process for producing Ti by Ca reduction.

As a further process for producing Ti, the Olson's process is described in the specification of U.S. Pat. No. 2,845,386 (hereinafter referred to as “Document 2”) which comprises directly reducing TiO₂ with Ca without going through TiCl₄ processing This process is a kind of direct oxide reduction method. However, the use of high-purity TiO₂, which is expensive, is inevitable in this process.

On the other hand, the present inventors considered that the reduction of TiCl₄ with Ca should be essential for the establishment of an industrial process for producing Ti by Ca reduction and that it would be necessary to economically replenish the Ca in the molten salt consumed in the reduction reaction, and they proposed, in Japanese Patent Application Publication No. 2005-133195 (hereinafter referred to as “Document 3”) and Japanese Patent Application Publication No. 2005-133196 (hereinafter referred to as “Document 4”), a process which utilizes the Ca formed by electrolysis of molten CaCl₂ and recycling this Ca, namely “OYIK process”. Document 3 cited above describes a process in which Ca is formed and replenished by electrolysis and the Ca-enriched molten CaCl₂ is introduced into a reaction vessel and used for the formation of Ti particles by Ca reduction, and Document 4 cited above further discloses a method of effectively inhibiting the back reaction resulting from electrolysis through the use of an alloy electrode (e.g. Mg—Ca alloy electrode) as a cathode.

DISCLOSURE OF INVENTION

As mentioned above, a number of research and development works have so far been made concerning processes for producing Ti other than the Kroll process. In particular, in the OYIK process proposed by the present inventors, Ca in the molten salt is consumed with the progress of the reduction reaction of TiCl₄ but when that molten salt is electrolyzed, Ca is formed in the molten salt; when the thus-obtained Ca is reused in the reduction reaction, Ca replenishment from outside becomes unnecessary and, furthermore, it is unnecessary to isolate and discharge Ca singly so that the economical efficiency is enhanced.

Accordingly, the present inventors made investigations concerning the step of electrolyzing molten CaCl₂ as part of the efforts to further develop a process for producing metallic Ti, wherein the core concept thereof is based on the OYIK process and the operation can be carried out more efficiently and reliably. The process for producing Ti or Ti alloys according to the present invention is named “OYIK-II process” after the initials of the four persons “Ogasawara, Yamaguchi, Ichihashi and Kanazawa” who had been deeply involved in coming up with an idea, development and completion of that process.

It is an object of the present invention to provide: a method for electrolyzing molten salt which makes it possible to carry out the recovery of a highly concentrated Ca-containing molten salt in obtaining the molten salt increased in Ca concentration by electrolyzing the molten salt containing chloride of a metal fog forming metal such as Ca, Li, Na or Al, in particular CaCl₂, and by which high current efficiency can be maintained and a large amount of molten CaCl₂ can be electrolyzed in a continuous manner; an electrolytic cell for use in carrying out such method; and a process for producing Ti by applying that method.

To accomplish the above object, the present inventors made detailed investigations concerning the shape of the electrolytic cell container, the shape of the electrode, the electrolysis conditions and the distance between electrodes, among others, using molten CaCl₂ and, as a result, have now completed the present invention.

The gist of the present invention consists in (1) a method for electrolyzing molten salt, (2) an electrolytic cell, and (3) a process for producing Ti using that method, as defined below.

(1) A method for electrolyzing a molten salt by which the electrolysis is carried out in such a state that molten salt containing the chloride of metal-fog forming metal is supplied from one end of an electrolytic cell to a space between an anode and a cathode in a continuous or intermittent manner to provide a flow rate in one direction to the molten salt in the vicinity of the surface of the cathode and thus to allow the molten salt to be electrolyzed in the vicinity of the surface of the cathode, while allowing flowing in one direction therein, to thereby enhance the metal-fog forming metal concentration.

The “metal-fog forming metal” so referred to herein is the metal capable of being itself dissolved in metal chloride, such as Ca, Li, Na or Al (namely, Ca is soluble in CaCl₂ or Li is soluble in LiCl₂), and capable of reducing TiCl₄.

When, in applying the method for electrolyzing molten salt, an electrolytic cell, in which the surfaces of the anode and cathode are disposed facing each other in a substantially vertical direction and a diaphragm or a partition wall configured to allow part of the molten salt to communicate therethrough is provided between the anode and cathode, is used, the recovery of chlorine gas generated on the anode side becomes facilitated. Furthermore, the back reaction which is the reaction between the metal-fog forming metal (e.g. Ca) formed by electrolysis and chlorine (Cl) to again form CaCl₂ can be prevented; hence the use of such electrolytic cell is preferable (hereinafter referred to as “a first mode of embodiment”).

When an embodiment mode is employed such that: the cathode is hollow and has gaps or holes; the molten salt can flow therethrough from the surface of the cathode into the inside thereof; and the molten salt enriched with metal-fog forming metal, which flows into the inside of the cathode, can be drawn out of the electrolytic cell, it becomes possible to effectively inhibit the back reaction (hereinafter referred to as “a second mode of embodiment”).

When the metal-fog forming metal concentration in the molten salt in the electrolytic cell is controlled so that it may be at a level lower than the saturation solubility, it becomes possible to increase the Ca concentration and increase the rate of formation of Ti and, further, prevent clogging in the inside of the electrolytic cell and like troubles (hereinafter referred to as “a third mode of embodiment”).

(2) An electrolytic cell which comprises an electrolytic cell container elongated in one direction and intended for retaining the molten salt containing the chloride of metal-fog forming metal, and an anode and a cathode disposed along the lengthwise direction of the electrolytic cell container, and is provided with a molten salt feeding port at one end of the lengthwise direction of the electrolytic cell container for supplying the molten salt to a space between the anode and cathode and with a molten salt drawing out port at the other end thereof for drawing out the molten salt increased in Ca concentration as formed by electrolysis of the molten salt.

When the electrolytic cell is configured such that the surfaces of the anode and cathode are disposed facing each other in a substantially vertical direction and, further, a diaphragm or a partition wall allowing part of the molten salt to communicate therethrough is disposed between the anode and cathode, it can suitably be used in applying the method of electrolysis according to the above-mentioned first mode of embodiment.

(3) A process for producing Ti which comprises: a reduction step of causing TiCl₄ to react with Ca in molten salt containing CaCl₂ and further Ca dissolved therein to thereby cause formation of Ti particles in the molten salt; a separation step of separating the Ti particles formed in the molten salt therefrom; and an electrolysis step of electrolyzing the molten salt decreased in Ca concentration in association with formation of Ti particles to thereby increase the Ca concentration, wherein the molten salt increased in Ca concentration as formed in the electrolysis step is used for reduction of TiCl₄ in the reduction step, and wherein the method for electrolyzing molten salt as defined above in (1) is applied in the electrolysis step.

The method for electrolyzing molten salt according to the present invention is the one comprising electrolyzing molten salt while the molten salt is caused to flow in one direction in the vicinity of the surface of the cathode and recovering the molten salt enhanced in metal-fog forming metal concentration on the outlet side of the electrolytic cell. This electrolysis method makes it possible to suppress the back reaction and maintain high current efficiency and, at the same time, effectively take out only the molten salt enriched in such a metal-fog forming metal as Ca and, further, continuously electrolyze a large amount of molten CaCl₂. This method can be applied with ease using the electrolytic cell according to the present invention.

Further, when the method for electrolyzing molten salt according to the present invention is applied to the production of Ti by Ca reduction, a molten salt enriched in Ca can be obtained in a relatively stable manner, so that metallic Ti can be produced efficiently.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a vertical sectional view illustrating a constitutional example of the principal parts of an electrolytic cell according to the present invention.

FIG. 2 is a partial schematic representation of another constitutional example of an electrolytic cell according to the present invention in which a hollow cathode is used.

FIG. 3 is a diagram showing, by way of example, the steps in applying the process for producing Ti according to the present invention.

BEST MODES FOR CARRYING OUT THE INVENTION

In the following, the method for electrolyzing molten salt, the electrolytic cell and the process for producing Ti using that method, each according to the present invention, are described more specifically, with reference to the drawings. Since, when forming Ti by reduction of TiCl₄ using the method for electrolyzing molten salt according to the present invention, all metal-fog forming metals act and behave in the same manner, the case where the metal fog forming metal is Ca is described in the following.

FIG. 1 is a vertical sectional view illustrating a constitutional example of the principal parts of an electrolytic cell to be used in applying the method for electrolyzing molten salt according to the present invention.

This electrolytic cell 1 comprises: an electrolytic cell container 1 a having a tubular (cylindrical) form elongated in one direction for retaining CaCl₂-containing molten salt; a similarly cylindrical anode 2 and a round column-like cathode 3 disposed in the container 1 a along the lengthwise direction of the electrolytic cell container 1 a; a molten salt feeding port 6 at one end (bottom 4) of the lengthwise direction of the electrolytic cell container 1 a; and a molten salt drawing out port 7 at the other end (upper wall 5) thereof. The surfaces of the anode and cathode are disposed facing each other in a substantially vertical direction and, further, a diaphragm 8 for preventing the passage of Ca formed by electrolysis of the molten salt is provided between the anode 2 and cathode 3. Furthermore, the outer surface of the anode 2 is provided with a cooling device 9.

The method for electrolyzing molten salt according to the present invention is characterized in that the molten salt containing the chloride (CaCl₂) of a metal-fog forming metal (Ca) is supplied from one end of the electrolytic cell to a space between the anode and cathode in a continuous or intermittent manner to thereby provide a flow rate in one direction to the molten salt in the vicinity of the surface of the cathode and the Ca concentration in the molten salt is increased by carrying out the electrolysis while the molten salt is caused to flow in one direction in the vicinity of the surface of the cathode.

Thus, according to the method for electrolyzing molten salt according to the present invention, CaCl₂-containing molten salt is first supplied from one end of the electrolytic cell 1 to a space between the anode 2 and cathode 3 in a continuous or intermittent manner. The “CaCl₂-containing molten salt” so referred to herein indicates molten CaCl₂ alone or molten salt with additive, to molten CaCl₂, such as KCl, CaF₂ or the like for lowering the melting point and adjusting the viscosity and so forth. Hereinafter, such molten salt is referred to simply as “molten salt”.

Since the electrolytic cell 1 has a shape elongated in one direction (in the example shown, a vertically elongated tubular (cylindrical) shape), it is possible to provide a flow rate in one direction to the molten salt in the vicinity of the surface of the cathode 3 and thereby cause the molten salt to flow in one direction in the vicinity of the surface of the cathode 3 by supplying the molten salt from one end of the electrolytic cell 1 to a space between the anode 2 and cathode 3 in a continuous or intermittent manner. In this case, it is enough to attain such a condition that at least a partial molten salt staying in the vicinity of the surface of the cathode 3 flows in one direction, or the whole molten salt between the anode 2 and cathode 3 may bodily flow in one direction. The phrase “in the vicinity of the surface of the cathode” refers to the region adjacent to the surface of the cathode where Ca formed on the surface of the cathode exists.

Although the molten salt is generally supplied continuously, the molten salt may be fed intermittently in relation to the subsequent step, for instance; namely, the supply of the molten salt may be temporarily suspended and then resumed again. When the molten salt supply is temporarily suspended, the flow of the molten salt in the vicinity of the surface of the cathode is also suspended. Therefore, the “flow rate” on the occasion of “providing a flow rate in one direction to the molten salt in the vicinity of the surface of the cathode”in the strict sense of the term includes also the no-flow condition in which the flow rate is 0 (zero).

The molten salt is then electrolyzed. That is, the molten salt is electrolyzed to form Ca on the surface of the cathode while it is caused to flow in one direction in the vicinity of the surface of the cathode. Since the electrolytic cell 1 has a shape elongated in one direction and, in the example shown in FIG. 1, the distance between the anode 2 and cathode 3 is relatively short to suppress the electrolytic voltage, the Ca-enriched molten salt alone can be drawn out effectively while the molten salt in proximity to the molten salt feeding port 6, which is low in Ca concentration, is prevented from mixing with the molten salt in proximity to the molten salt drawing out port 7, which has an increased Ca concentration as a result of electrolysis.

The technology described in Document 2 cited above uses Ca as the reducing agent but is a direct reduction method for reducing TiO₂, not TiCl₄, with Ca to Ti and thus is different from the method of electrolysis according to the present invention. Furthermore, in the direct reduction method described in Document 2 cited above, the carbon electrode to be used as the anode is consumed as CO₂ and, in addition, titanium carbide (TiC) is formed in the molten salt, so that the resulting Ti is adulterated with C-contaminated Ti and the workability is deteriorated; problems may be encountered in using such Ti for making wrought products.

Further, Document 2 cited above describes a technology of “forming a flow of molten salt in the vicinity of the cathode in forming Ti by Ca reduction in molten salt”. However, there is no description suggesting the technological philosophy of the present invention that: the anode and cathode should be disposed facing each other along the lengthwise direction of the electrolytic cell; the molten salt should be caused to flow in one direction in the vicinity of the surface of the cathode or, where a diaphragm or the like is provided, in the cathode compartment formed between the surface of the cathode and the diaphragm; and the electrolysis should be carried out under such conditions to thereby recover the molten salt increased in Ca concentration on the outlet side of the electrolytic cell.

Therefore, the method for electrolyzing molten salt according to the present invention and the technology described in Document 2 are quite different from each other even if they are common in that the molten salt is caused to form a unidirectional flow in the electrolytic cell.

In the first mode of embodiment thereof, the method for electrolyzing molten salt according to the present invention is the one using an electrolytic cell in which the surfaces of the anode and cathode are disposed facing each other in a substantially vertical direction and a diaphragm or a partition wall configured so that part of the molten salt can communicate therethrough is provided between the anode and cathode. The term “substantially” in the above-mentioned phrase “in a substantially vertical direction” means “almost” or “approximately”, and the “substantially vertical direction” indicates the vertical direction or a direction slightly slanted from the vertical direction.

The electrolysis method in the first mode of embodiment can be carried out more preferably by using such an electrolytic cell as shown by way of example in FIG. 1. While the system employed in the electrolytic cell shown in FIG. 1 comprises feeding CaCl₂ from the lower side of the electrolytic cell 1 and drawing out the same from the upper side thereof, it is also possible to employ the system comprising supplying CaCl₂ from the upper side of the electrolytic cell 1 and drawing out the same from the lower side thereof.

In the electrolytic cell to be used in this electrolysis method, the surfaces of the anode and cathode are disposed facing each other in a substantially vertical direction and, on the other hand, the molten salt in the vicinity of the surface of the cathode is provided with a flow rate in one direction and the direction of the flow of the molten salt is vertical, hence chlorine gas generated on the anode side easily moves upwards to the surface and can be recovered with ease.

Usable as the diaphragm to be disposed between the anode and cathode is, for example, a porous ceramic body comprising yttria (Y₂O₃). A porous ceramic body prepared by firing yttria has selective permeability such that ions such as Ca and chlorine can permeate therethrough but metallic Ca cannot and, further, has good resistance to calcium reduction such that it cannot be reduced even with Ca strong in reducing power Therefore, the porous ceramic body is suited for use as the diaphragm in applying the method for electrolyzing molten salt according to the present invention.

When an electrolytic cell provided with such a diaphragm between the anode and cathode is used, the back reaction, namely the immediate reaction between the Ca formed on the cathode side and the chlorine formed on the anode (graphite) side to again form CaCl₂, hardly occurs and the electrolysis can be carried out with high current efficiency.

A partition wall configured so that part of the molten salt can communicate therethrough may be used instead of the diaphragm. The partition wall does not allow the passage of not only metallic Ca but also such molten salt constituents as Ca and chlorine ions but, when the partition wall is partially provided with slits or holes through which the molten salt can communicate, it enables the electrolysis and, on the other hand, restricts the passage of metallic Ca to a certain extent, making it possible to suppress the back reaction.

The second mode of embodiment of the method for electrolyzing molten salt (including the first mode of embodiment) according to the present invention is the one by which the cathode is hollow and has gaps or holes through which the molten salt can. flow from the surface of the cathode into the inside thereof (i.e. inner hollow space) so that the Ca-enriched molten salt that flew into the inside of the cathode can be drawn out of the electrolytic cell.

FIG. 2 is a partial schematic representation of another constitutional example of an electrolytic cell in which a hollow cathode is used. As shown in FIG. 2, in this electrolytic cell 1, an anode 2 and a hollow cathode 3 a are disposed facing each other in a substantially vertical direction along the lengthwise direction of the electrolytic cell 1 and a diaphragm 8 is provided between the anode 2 and cathode 3 a. The cathode 3 a is provided with gaps or holes (not shown) through which the molten salt can flow from the surface of the cathode into the inside thereof.

When a thus-constituted electrolytic cell is used and the molten salt is drawn out from the upper side of the hollow section of the cathode 3 a, a flow of the molten salt from the outer surface of the cathode to the inside thereof (inner hollow space) is formed, as shown by outlined arrows, and the Ca formed on the outer surface of the cathode 3 a is directly taken into the inside of the cathode 3 a without dispersion or migration to the anode side. As a result, the back reaction can be effectively prevented. The electrolytic cell shown by way of example in FIG. 2 comprises a diaphragm 8, so that the back reaction preventing effect is much stronger as compared with the case where there is no diaphragm.

The size and positions, among others, of the gaps or holes to be provided in the hollow cathode are not particularly limited. They may properly be selected so that an effective molten salt flow toward the inner surface of the cathode may be formed, taking into consideration the distance between the surface of the anode (the diaphragm surface when a diaphragm is provided) and the outer surface of the cathode, the amount of the molten salt drawn out (amount of the molten salt supplied) and other factors.

The third mode of embodiment of the method for electrolyzing molten salt (including the first and second modes of embodiment) according to the present invention is the one of electrolysis by which the Ca concentration in the molten salt in the electrolytic cell is controlled so that it may be at a level lower than the saturation solubility. By saying “the Ca concentration is controlled so that it may be at a level lower than the saturation solubility” above, it is meant that the electrolysis is carried out “under conditions such that the Ca concentration should be close to the saturation solubility but should not be so much to allow Ca to precipitate out”.

More specifically, optimum electrolysis conditions, an amount of molten salt to be drawn out per unit time and other factors are determined empirically according to the shape of the electrolytic cell container, the shapes of the electrodes, the distance between poles and the like, so that “the conditions that the Ca concentration should be close to the saturation solubility but should not be so much to allow Ca to precipitate out” may be satisfied at the site showing the maximum Ca concentration in the electrolytic cell. In particular, when a diaphragm or partition wall is used between the anode and cathode, the Ca concentration becomes maximum in proximity to the molten salt drawing out port on the cathode side. Therefore, by controlling the Ca concentration at such site at a level lower than the saturation solubility, the electrolytic operation may be carried out without allowing metallic Ca to precipitate out at any site in the electrolytic cell.

When such a electrolysis method is employed, it is possible to obtain the molten salt enriched in Ca to a level close to the saturation solubility in a relatively stable manner while such troubles as clogging in the inside of the electrolytic cell are prevented.

In an exemplary mode of embodiment of the Ca enrichment according to the present invention, when the temperature of CaCl₂ entering the electrolytic cell may be set at 800° C., the metallic Ca concentration in the molten CaCl₂ can be increased from 0% to a metallic Ca concentration of 1% in the molten CaCl₂ leaving the electrolytic cell. It is preferable that the metallic Ca concentration (concentration A) in the molten CaCl₂ entering the electrolytic cell is from 0% to less than 1% and the metallic Ca concentration (concentration B) in the molten CaCl₂ leaving the electrolytic cell be not less than 0.1%. Considering the efficient utilization of Ca in the subsequent step, the increment (concentration B-A) in metallic Ca concentration in the electrolytic cell is preferably not less than 0.1% and not more than 5.0% (concentration including supersaturated Ca), particularly preferably not less than 1.0%.

In carrying out the method for electrolyzing molten salt according to the present invention, the heat of reaction is generated in large quantities in the electrolytic cell and therefore it is preferable that the heat is removed effectively. More specifically, either in cases where the hollow cathode mentioned above is used or in cases where such is not used, it is preferable that a cooling device is disposed in the central part of the cathode to remove the heat of reaction from the inside of the cathode. A tubular heat exchanger, for instance, is suited for use as the cooling device.

When a cooling device (heat exchanger) is disposed on the anode side, the heat removal efficiency is further enhanced. The cooling device 9 disposed so as to surround the anode 2 as shown in FIG. 1 is an example.

For increasing the yield of Ca by increasing the current supply in electrolysis, it is necessary to enlarge the surface area for current supply. It is preferable that the inside surface of the anode 2, namely the surface opposing the surface of the cathode in the electrolytic cell 1 shown by way of example in FIG. 1, is provided with minute concavo-convex irregularities to secure a large surface area for current supply. Applicable as the means therefor is, for example, grooving for forming grooves on the electrode surface.

In accordance with the method for electrolyzing molten salt according to the present invention, since the electrolysis is carried out while the molten salt is caused to flow in one direction in the vicinity of the surface of the cathode, a large amount of the molten salt can be treated continuously.

The electrolytic cell according to the present invention is an electrolytic cell to be used in carrying out the above-mentioned method for electrolyzing a molten salt and is characterized in that it comprises: an electrolytic cell container elongated in one direction and intended for retaining a molten salt containing CaCl₂; an anode and a cathode disposed along the lengthwise direction of the electrolytic cell container; a molten salt feeding port at one end of the lengthwise direction of the electrolytic cell container for supplying the molten salt to a space between the anode and cathode; and a molten salt drawing out port at the other end thereof for drawing out the molten salt increased in Ca concentration as formed by electrolysis of the molten salt.

The electrolytic cell, shown by way of example in FIG. 1, is one mode of embodiment of the electrolytic cell according to the present invention, where the surfaces of the anode and cathode are disposed facing each other in a substantially vertical direction and a diaphragm is disposed between the anode and cathode. A partition wall configured so that part of the molten salt can communicate therethrough may be disposed therein instead of the diaphragm. When the electrolytic cell shown in FIG. 1 is used, the method for electrolyzing molten salt according to the present invention can be suitably applied, as already mentioned hereinabove.

The process for producing Ti according to the present invention characterized by comprising: a reduction step of causing TiCl₄ to react with Ca in a molten salt containing CaCl₂ and further Ca dissolved therein to thereby cause formation of Ti particles in the molten salt; a separation step of separating the Ti particles formed in the molten salt therefrom; and an electrolysis step of electrolyzing the molten salt decreased in Ca concentration in association with formation of Ti particles to increase the Ca concentration, wherein the molten salt increased in Ca concentration as formed in the electrolysis step is used for the reduction of TiCl₄ in the reduction step, and wherein the method for electrolyzing the molten salt according to the present invention is applied in the above-mentioned electrolysis step.

FIG. 3 is a diagram showing, by way of example, the steps in applying the process for producing Ti according to the present invention. As shown in FIG. 3, this process for producing Ti comprises: the reduction step 10 of causing TiCl₄ to react with Ca in a molten salt containing CaCl₂ and further Ca dissolved therein to form Ti particles in the molten salt; the separation step 11 of separating the Ti particles formed in the molten salt from the molten salt; and the electrolysis step of electrolyzing the molten salt reduced in Ca concentration in association with formation of the Ti particles to increase the Ca concentration. In the process for producing Ti according to the present invention, the above-mentioned method for electrolyzing the molten salt is applied in this electrolysis step and, therefore, an electrolytic cell 1 for use in this electrolysis step is included therein.

The electrolytic cell 1 used here comprises: an electrolytic cell container 1 a having a vertically elongated cylindrical shape; an anode 2 and a cathode 3 disposed along the lengthwise direction of the electrolytic cell container 1 a; and a diaphragm 8 disposed between the anode 2 and cathode 3. The electrolytic cell 1 is provided, at the upper end thereof, with a molten salt feeding port (not shown) for supplying molten salt to a space between the anode 2 and cathode 3 and, at the lower end thereof, with a molten salt drawing out port (not shown) for drawing out the molten salt increased in Ca concentration as formed by electrolysis of the molten salt.

The CaCl₂-containing molten salt supplied from the upper end of the electrolytic cell 1 moves downward within the electrolytic cell and electrolyzed during movement, whereby Ca is formed. The Ca concentration in the molten salt is increased as the molten salt moves downward. During electrolysis, the back reaction is suppressed by the diaphragm 8 disposed between the anode 2 and cathode 3 and, thus, the current efficiency is maintained at a high level. Further, during operation, the Ca concentration in the molten salt is controlled at a level lower than the saturation solubility, namely so that the Ca concentration may be close to the saturation solubility but not so much to allow Ca to precipitate out. In addition, since the electrolytic cell 1 is a vertical type, the chlorine gas generated on the anode side can be recovered with ease.

The thus-obtained molten salt enriched in Ca is drawn out through the molten salt drawing out port at the lower end of the electrolytic cell 1 and transferred to the reduction step 10. In the reduction step 10, TiCl₄ gas is caused to react with Ca in the molten salt enriched in Ca, whereby granular metallic Ti is formed in the molten salt. As the reduction reaction proceeds in the molten salt, the Ca in the molten salt is consumed while Ti is formed and at the same time CaCl₂ is formed as a byproduct.

The Ti particles formed in the reduction step 10 are transferred, together with the molten salt, to the separation step 11, and the Ti particles are separated from the molten salt. Applicable to the separation are a solid-liquid separation procedure using such as a high-speed decanter (continuous centrifugation) system, a thickener system or the like. If, though not shown, the reaction vessel to be used in this reduction step 10 is constituted so that the byproduct CaCl₂-containing molten salt may be discharged out of the vessel, it is also possible to transfer the molten salt discharged from this reduction step 10 directly to the electrolysis step (cf. Documents 3 and 4 cited above).

While the Ti powder obtained in the Kroll process is in an agglomerated state, the Ti particles obtained in the reduction step 10 is hardly agglomerated and is hardly adhering to the vessel, so that they are easily taken out of the vessel; the recovered Ti particles, as such, can be transferred to the melting step in which they are heated and melted to provide a Ti ingot 12.

On the other hand, the remaining molten salt reduced in Ca concentration after separation and recovery of the Ti particles is sent to the electrolysis step, in which it is subjected to electrolysis treatment in the above-mentioned electrolytic cell 1 and the resulting molten salt enriched in Ca is again used for reducing TiCl₄ in the reduction step 10.

In the process for producing Ti according to the present invention, the molten salt enriched in Ca to a level close to the saturation solubility is obtained in the electrolysis step in a relatively stable manner, so that metallic Ti can be produced with good efficiency; and, further, Ca formed by continuous electrolysis of a large amount of molten salt can be supplied to the reduction step. Therefore, the process can be also suited for mass production.

INDUSTRIAL APPLICABILITY

The method for electrolyzing molten salt according to the present invention is the one for carrying out electrolysis while the molten salt is caused to flow in one direction in the vicinity of the surface of the cathode and, according to this method of electrolysis, high current efficiency can be maintained and only the molten salt enriched in such metal-fog forming metal as Ca can be taken out effectively. This electrolysis method can be carried out with ease using the electrolytic cell according to the present invention. Further, when the method for electrolyzing molten salt according to the present invention is applied to the production of Ti by Ca reduction, a Ca-enriched molten salt is obtained in a relatively stable manner and metallic Ti can be produced with good efficiency. Therefore, the method for electrolyzing molten salt, the electrolytic cell, and the process for producing Ti in which said electrolysis method is applied, each according to the present invention, can be effectively utilized in the production of Ti by Ca reduction. 

1-7. (canceled)
 8. A method for electrolyzing molten salt employing an electrolytic cell provided, between an anode and a cathode, with a diaphragm or a partition wall configured so that part of the molten salt can communicate therethrough, comprising: supplying molten salt containing the chloride of a metal-fog forming metal to a space between said cathode and said diaphragm or partition wall in a continuous or intermittent manner; and, providing a flow rate in one direction to the molten salt in the vicinity of the surface of the cathode to thereby cause the formation of a flow of the molten salt in one direction along the surface of the cathode, and carrying out the electrolyzing along the flow to thereby enhance the concentration of the metal-fog forming metal.
 9. The method for electrolyzing a molten salt according to claim 8, in which the method entails employing an electrolytic cell comprised of an anode and a cathode where the surfaces of said anode and cathode are disposed facing each other in a substantially vertical direction.
 10. The method for electrolyzing molten salt according to claim 8, wherein the concentration of the metal-fog forming metal in the molten salt in the electrolytic cell is controlled so that it may be at a level lower than the saturation solubility.
 11. An electrolytic cell, comprising: an electrolytic cell container elongated in one direction and intended for retaining molten salt containing the chloride of the metal-fog forming metal; an anode and a cathode disposed along the lengthwise direction of the electrolytic cell container; a diaphragm or a partition wall, being configured so that part of the molten salt can communicate therethrough, as disposed between said anode and said cathode; a molten salt feeding port capable of supplying the molten salt to a space between said cathode and said diaphragm or said partition wall, as provided at one end of the electrolytic cell container; and a molten salt drawing out port provided at the other end thereof for drawing out the molten salt increased in Ca concentration as formed by electrolysis of the molten salt.
 12. The electrolytic cell according to claim 11, wherein the surfaces of said anode and said cathode are disposed facing each other in a substantially vertical direction.
 13. A process for producing Ti, comprising: a reduction step of causing TiCl₄ to react with Ca in a molten salt containing CaCl₂ and further Ca dissolved therein to thereby cause formation of Ti particles in said molten salt; a separation step of separating said Ti particles formed in said molten salt from said molten salt; and an electrolysis step of electrolyzing molten salt decreased in Ca concentration in association with formation of Ti particles to increase the Ca concentration, wherein the molten salt increased in Ca concentration as formed in the electrolysis step is used for reduction of TiCl₄ in the reduction step, and wherein the method for electrolyzing molten salt according to claim 8 is applied in said electrolysis step.
 14. The method for electrolyzing molten salt according to claim 9, wherein the concentration of the metal-fog forming metal in the molten salt in the electrolytic cell is controlled so that it may be at a level lower than the saturation solubility. 